1. Field of the Invention
The present invention relates to a display device, and particularly to a projection display apparatus using a reflective display device.
2. Description of the Related Art
Hitherto, a thin-film micromirror array (TMA) or the like is known as a display device for displaying an image by linearly varying an angle of a reflection mirror, and is disclosed in Japanese Patent Laid-Open No. 11-072724, Japanese Patent Laid-Open (by PCT route) No. 10-508953, and so on.
Also, a digital micromirror device (DMD) or the like is known as a display device in which a display element is driven by converting a high-gradation picture signal into a binary pulse submodulating signal in accordance with a time-division drive sequence, so as to perform high-gradation display, and is disclosed in Japanese Patent Laid-Open Nos. 8-214243 and 8-214244, and so on.
In a display device using a conventional linear driving method, however, brightness nonuniformity occurs in a display due to differences of reflection angles resulting from manufacturing variation of each display cell, thus causing deterioration of a display quality. Further, a D/A converter having the number of bits corresponding to the number of display gradations is required. This results in a problem that size of a circuit is large and cost is high.
When a display element is driven by converting a high-gradation picture signal into a binary pulse submodulating signal, display resolution is limited by the response speed and data transfer rate of a display device.
Accordingly, it is an object of the present invention to provide a display device, a projection display apparatus, a driving device for a light modulator, and a method for driving the light modulator, in which high-gradation is achieved while deterioration of display quality due to manufacturing variation can be overcome.
In order to achieve the object, the present invention provides a display device comprising a light modulator, wherein gradation is represented by varying the ON-time in the temporal axis during one frame period of the light modulator, and the ON-time is used by at least three light components.
The one frame period may comprise binary subframes for performing black/white display and a multilevel subframe for performing gradation display.
The present invention also provides a projection display apparatus comprising a light modulator used as image forming means and a projection optical system for projecting and displaying an image, wherein gradation is represented by varying the ON-time in the temporal axis during one frame period of the light modulator, and the ON-time is used by at least three light components.
Preferably, the light modulator is a reflective device including a micromirror, and the gradation is performed by controlling the micromirror to at least three reflection angles including a first reflection angle in which the amount of light reflection to the projection optical system is the maximum; a second reflection angle in which the amount of light reflection to the projection optical system is zero; and a third reflection angle between the first reflection angle and the second reflection angle in which the amount is between the amount for the first reflection angle and the amount for the second reflection angle.
It is desired that, in representing gradation by varying the ON-time in the temporal axis during one frame period, the light modulator has a first display period in which the second reflection angle and the first reflection angle are mutually switched for displaying, and a second display period in which the second reflection angle and the first reflection angle or the third reflection angle are mutually switched for displaying.
The second display period may the shortest period among a plurality of subframe periods in a time-division gradation representation of one frame period.
Preferably, in representing gradation by varying the ON-time in the temporal axis during one frame period, the light modulator has a first display period in which the second reflection angle and the first reflection angle are mutually switched for displaying, and a second display period in which the second reflection angle and the first reflection angle are mutually switched for displaying.
It is desired that the first display period and the second display period are allotted so that the total number of subframe periods for performing gradation by time-dividing one frame period is minimized.
According to the present invention, it becomes possible to reduce brightness nonuniformity in the display, which results from manufacturing variations of each display cell, which occurred in a conventional linear driving method. Conventional methods require a D/A converter having the number of bits corresponding to the number of display gradations. In contrast, the present invention requires a D/A converter having only the number of bits corresponding to the number of gradations represented by the subframe of the minimum period, hence, size of the circuit is reduced and low cost can be achieved.
By using a projection display apparatus, a driving device for a light modulator, and a method for driving the light modulator of a second embodiment of the present invention, the response speed needed for the device can be reduced, and the data transfer rate and transfer clock frequency can be suppressed.
The present invention also provides a method for driving a light modulator, comprising: dividing one frame period of the light modulator into subframe periods which have different periods corresponding to each bit; performing gradation display by at least ternary digits in the light modulator during the minimum subframe period; and performing binary gradation display during subframe periods other than the minimum subframe period.
The present invention also provides a method for driving a light modulator, comprising: performing a first driving state for white displaying, a second driving state for black displaying, and a third driving state for gradation display between the white displaying and the black displaying; setting the amount of light reflection in the third driving state to be small by a predetermined proportion to the amount of light reflection in the first driving state; dividing one frame period of the light modulator into subframe periods which have different periods corresponding to each bit; allotting any one of the first driving state and the third driving state to each of the subframe periods; defining a time slot corresponding to the minimum subframe period as a unit time slot; and reducing by the predetermined proportion the number of time slots in the subframe periods to which the first driving state is allotted, so as to reduce the total number of time slots in the one frame period, thereby making a unit time slot period, which is the one frame period divided by the reduced total number of time slots, longer than a unit time slot period, which is the one frame period divided by the unreduced total number of time slots.